1. Field of the Invention
This invention relates to shift events of automotive transmissions and more particularly relates to improving the rate of deceleration of an engine to improve shift quality of automated manual transmissions.
2. Description of the Related Art
The purpose of an automotive transmission is to allow an engine to operate within a narrow range of speeds while outputting a broad range of speeds. Without a transmission, a vehicle would be limited to using one gear ratio. Vehicles need transmissions to take advantage of the narrow rpm ranges of the engine where horsepower and torque are at a maximum. The transmission allows the gear ratio between the engine and the drive wheels to change as the vehicle speeds up and slows down. Shifting gears allows the engine to stay near the rpm range that will provide the best performance.
At least three different types of transmissions have been developed over the past century to enable the engine to operate in a broad range of speeds. The first, the manual transmission, generally includes an output shaft driven by the engine, a layshaft, and a drive shaft that drives the wheels of the vehicle. Each shaft has at least one sprocket or gear that meshes with the gears of the other shafts. A gear selector moves a collar to engage the gears of the drive shaft. In many situations the manual transmission is ideal, such as a sports car. However, heavy duty trucks often require 10 or more gears, and shifting through each of those gears to get the heavy duty truck up to speed requires skill and can be tiresome.
The second type of transmission is the automatic transmission. The automatic transmission, as the name suggests, automatically shifts between gears. The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, while in an automatic transmission, a planetary gearset produces all of the different gear ratios. Automatic transmissions greatly reduce the skill required to operate vehicles, and also improve the perceived “smoothness” of a vehicle by reducing rough shift events of inexperienced drivers. However, automatic transmissions are not able to transfer the large amounts of torque produced by the engines of heavy duty trucks.
The third type of transmission is an automated manual transmission. The automated manual transmission attempts to blend the ability of the manual transmission to handle large amounts of torque with the automatic transmission's ease of use. Automated manual transmissions include the traditional manual gearboxes, but also electronic controls and actuators that make all decisions of when to shift, as well as actually performing the shift. Shift maneuvers are performed when a transmission control module issues commands via an electronic datalink to an engine control module.
The automated manual transmission generally transitions smoothly from one gear to another. Overall vehicle acceleration performance is impacted by the total time spent at zero torque, or in other terms, out of gear. This is particularly important when accelerating on a steep grade. During the shift event, the vehicle is decelerating. This has the effect of moving the target synchronization speed down during the shift. The target synchronization speed is the ideal engine speed at which to shift from neutral into the next gear. For example, while the initial target engine speed may have been 1400 revolutions per minute (RPM) when the transmission first started the shift event, the time spent at zero engine torque causes the vehicle to slow and thereby cause the target synchronization speed to drop.
In certain situations the engine cannot decelerate quickly enough to “catch up” with the moving sychronization target before that target drops below the engine speed where, once back in gear, the engine could effectively accelerate the truck. In other words, by the time the engine reaches the target speed and allows the transmission to go into gear, the engine speed is too low to accelerate the truck in that gear. Such an event causes a rough shift event, noticeable drive shaft ringing or vibrations, and causes undue wear on the engine and transmission.
To improve this situation, some automated manual transmissions will request engine brakes to increase the rate of deceleration of the engine. This has the effect of reducing the time it takes to reach the target synchronization speed and allows the engine to “catch” the target at a speed at which the engine will have enough torque to accelerate the vehicle once the transmission goes into gear. However, the use of engine brakes brings about new performance challenges.
Engine brakes require a few hundred milliseconds to activate. Therefore, even when engine brakes are requested, the engine initially decelerates at the natural rate, and subsequently extremely fast once the engine brakes have activated. The use of engine brakes can cause a rate of deceleration of about 2500 RPM per second. The combination of this extreme deceleration rate and engine brake hydromechanical deactivation delays make it difficult for the engine governor to catch the engine at the target synchronization speed and often cause the engine to “overshoot” the target synchronization speed. This results in rough shift events and undesired driveline component abuse.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method that improve the rate of deceleration of an engine. Beneficially, such an apparatus, system, and method would improve the rate of deceleration of the engine without the use of engine brakes, and if engine brakes are required by the transmission control module, only utilize a portion of the engine brakes.